1. Field of the Invention
The present invention relates to a method for generating a relief print on a substantially flat substrate, wherein palpable differences exist between a height of various parts of the relief print, by a printer comprising a number of colorants including a white colorant, said printer being configured to print a relief print in a number of passes. The present invention further relates to a computer program product, including computer readable code embodied on a computer readable medium, said computer readable code comprising instructions for generating a relief print and to a print system configured to generate relief prints.
2. Description of the Related Art
Print systems usually apply colorants on a substrate in the form of toner or ink according to a digitally defined, two-dimensional pattern of pixels with values that indicate a composition of these colorants. This pattern is generated out of a digital image, that may comprise objects in either vectorized or rasterized format, using conventional techniques like interpretation, rendering, and screening by a raster image processor. The processing of a digital image includes color management to convert color values of the pixels in the digital image into composition values related to the printer color space as is set up by the colorants of the print system. Depending on the intended print quality and the characteristics of the print process, the pixels of the pattern may be printed in more than one pass, wherein a position on the substrate has an opportunity to receive a colorant in one or more of the passes across the substrate. For every pass, a pass image is derived that contains pass pixels, which are the pixels for which a colorant is to be applied in that specific pass.
As an extension of the applications of these systems, a print may be made by stacking several colorants on the same substrate position on top of each other. The effect of this stacking depends on the properties of the colorant, but several colorants, such as UV curable ink or hot melt ink, will develop a height depending on the number of times a pixel is printed. Height differences between various pixels may create palpable differences between various parts of the print, giving the print a relief. Such prints are therefore termed relief prints. A pixel in a digital image that may lead to a relief print, is not only characterized by a color value, but additionally by a height value. The derivation of the pass images from a digital relief image, such that the relief print, made by successively printing the pass images on top of each other, is a physical representation of the relief image, is part of the rendering process in a relief print system.
Although a color and a height of the pixels in the digital image may be separately specified, there is a reciprocal relationship between the two. Printing a fixed colorant combination several times on top of each other makes the printed color in general more dark, thus limiting the obtainable gamut, which is the set of all possible colors that may be printed. In US Patent Application 2011/0193871, an accumulator of total consumed opacity is introduced to store an amount of opacity that is consumed by the already deposited layers. This opacity is allocated to the background on which a new layer is rasterized, which means that the colorant combination is adjusted along the height of the printed pixel. A disadvantage of this method is the reduction of the gamut as a result of the impossibility to generate the same colors on a dark background compared to the possible colors on a light background.
It is a desire to make relief prints with a varying height scale, keeping the pixel heights relative to each other constant. The absolute height scale is dependent on particular print conditions or parameter settings. A problem exists in obtaining a gamut of colors that is independent of the height of the pixels in a relief print. An object of the invention is to provide a method for deriving pass images for generating relief prints for which the gamut is independent of the height of the pixels.